My elderly neighbor has a 1998 Expedition with an alternator problem as all of the units I've installed and tested output less than 12v.
He wants to save some money and purchases used alternators from junkyards and winds up returning them. So far, I've installed 3 alternators and they each show different voltage readings with no loads: 11.75v, 11.82v, 11.88v. With a load (lights, A/C, etc.) the voltages drop into the 11.2v to 11.3v range, regardless of how much I rev the engine.
With the car off, the battery shows 12v and when the car is running, it will be less than 12v.
Are all the alternators bad or is there something else causing the low voltage problem? The power cable from the alternator to the battery is fine as is the negative (-) cable to the chassis.